The present invention relates generally to a system and method for monitoring DC link capacitance variance in three level inverters and more particularly to a system and method for injecting a signal into a neutral point regulator of a motor drive converter and measuring the response to the injected signal to monitor DC link capacitance.
Voltage source inverter drives for large AC motors incorporate large metalized film DC link capacitors. As these capacitors age, their capacitance decreases slowly. The gradual decreasing capacitance of these capacitors places an increasing burden on the neutral point clamped motor drive system and can lead to loss of function or to significant damage to the motor drive system. Additionally, when a catastrophic failure of a capacitor is experienced expensive and unexpected maintenance must be performed to replace the failed capacitor. Currently capacitors are replaced as part of a periodic maintenance program to avoid these problems. A more efficient system for avoiding failed capacitors is needed.
Three level neutral point clamped inverters are able to produce current waveforms with low amounts of harmonic distortion at higher power levels than conventional two level inverters. The switching states available in a three level neutral point clamped inverter allow regulation of the neutral point voltage assuming the presence of DC link capacitors in the power circuit. The capacitance value of the DC link capacitors determines the magnitude of the neutral point error. The neutral point error is one measure of the successful operation of the drive system, and if the error exceeds preset limits, the drive will cease operation.
The inventors have discovered that it is possible to monitor the capacitance of DC link capacitors in an inverter by injecting a signal into a neutral point regulator and measuring the response to the injected signal.
An embodiment of the present invention injects a signal into a neutral point regulator such as that which is disclosed in co-assigned U.S. Pat. No. 5,910,892 (hereafter the xe2x80x9c""892 patentxe2x80x9d) which is incorporated herein in its entirety. The ""892 patent discloses a high power motor drive converter having a three-level neutral point clamped output power conversion stage that includes control algorithms to balance the neutral voltage of a DC capacitor bank using space vector modulation and predictive charge calculations. The inventors of the present invention realized that the response to a signal injected into the neutral point regulator of the ""892 system can provide an indication of the DC link capacitance in the DC capacitor bank of the motor drive converter. The three-level inverter regulates the neutral potential by selecting switch states to regulate the neutral voltage to near zero. The invention modulates the neutral at a specified frequency and measures the neutral voltage and charge to determine the capacitance.
The present invention does not compromise the operation of the drive under load and does not require manual operator intervention to accomplish its task. No additional power supplies, PC boards, connectors, cables, etc. are required in the system so that costs, complexity and reliability are minimally impacted.